1. Field of the Invention
The present invention relates to a nozzle suitable for injection molding of resin of highly temperature-dependent viscosity such as liquid crystal polymer.
2. Detailed Description of the Prior Art
As molding materials of components for electronic equipment, resins such as liquid crystal polymer (LCP), high heat-resisting and low moisture-absorbing nylons (PA46), (PA6T), (PA9T), and polyphenylene-sulfide (PPS).
Since any of these resins has highly temperature-dependent viscosity and slight temperature lowering at the time of injection molding causes a sudden change in viscosity to affect on the fluidity, the temperature control of the resin is very important for stable molding.
In injection molding of liquid crystal polymer, the temperature of the nozzle main body is maintained at approximately 360xc2x0 C. at which the polymer has sufficient fluidity. However, the front end of nozzle comes into contact with a sprue bush of a low temperature metallic mold (for example, 80xc2x0 C.), and the front end of nozzle is deprived of heat and cooled down to approximately 270xc2x0 C.
Therefore, the resin in the nozzle port solidifies due to the temperature lowering of the front end of the nozzle, and becomes one piece with the sprue, and this is cleanly drawn out of the smallest diameter part on the boundary of the passage by mold opening after injection and cooling, to be removed from the nozzle port. Moreover, the melting resin is maintained in the passage inside of the boundary until the next injection without being extruded to the front end side.
When the temperature of the front end of nozzle is too high in such a state, the resin in the nozzle port is imperfectly solidified and cannot be drawn out of the boundary by mold opening, but is drawn out as it is torn off halfway and partly stays in the nozzle port. The remaining part is cooled down and solidified before the next injection and stays to choke the nozzle port. For this reason, the remaining part is injected into the metallic mold in the solidified state when infecting new resin.
When the temperature of the resin collected near the boundary of the passage is too high, the resin flows into the nozzle port before the next injection, and further flows into the sprue bush of the next metallic mold and cooled down for being solidified. Such outflow of the resin is called dropping, and if the solidified resin is injected at the next cycle, it is packed in a gate or narrow parts of a product and causes a short shot.
In order to avoid the dropping, the molding is carried out at a temperature lowered by 20xc2x0 C. at a molding site. However, resin of highly temperature-dependent viscosity is increased in viscosity in the flow passage and decreased in fluidity, and this causes difficulty in filling the resin into narrow cavity gap in molding of a molded article with thin walls and results in a short shot.
Temperature control is only the way to prevent such a short shot from occurring, and this means that the molding needs to be carried out under such a severe temperature control as the resin temperature at the boundary between the nozzle port and the passage has to be maintained in a temperature range not to cause dropping, and also has to be maintained in a temperature range not to cause lowering in fluidity.
Therefore, considering easiness of the temperature control and prevention of the dropping, the nozzle was divided into a nozzle tip and a nozzle main body; the resin was prevented from staying by forming the passage of the main body in a form of a tapered hole; such a dropping-preventive nozzle was developed by limiting the nozzle port diameter to a small one; and each kind of resins mentioned above was injection-molded into connectors by using an injection molding machine mounted with the nozzle, and as a result, it has been found out that the nozzle can be applied to many resins of highly temperature-dependent viscosity as far as the passage, the gradient of the nozzle port, the passage volume, etc. are within specific ranges.
Therefore, the purpose of the present invention is to provide a new nozzle for injection molding permitting to perform stable injection molding of even a small capacity product by making a nozzle, which is divided into the nozzle tip and the nozzle main body and in which the passage in the main body is formed of a tapered hole, as a nozzle for general use to be applicable to the resin of highly temperature-dependent viscosity by restricting the gradients of the passage and the nozzle port, the passage volume, etc. within specific ranges.
The present invention for the above-mentioned purpose is such that the nozzle comprises a cylindrical nozzle main body of which both center parts of the front end and the rear end are formed into thread shafts and the internal center passage is formed of a smoothly tapered hole with a gentle gradient, a cap shape nozzle tip which has a nozzle port of a tapered hole with a diameter of the inside smaller than a diameter of the outside and which is made of low thermal conductivity metal mounted on the nozzle main body by screwing the inside thread on the above-mentioned thread shaft on the front end, and a heating means on the outer periphery of the above-mentioned nozzle main body, and that the above-mentioned nozzle port diameter on the inside and the hole diameter at the front end of the passage of the above-mentioned nozzle main body are set within the range of 0.9-1.5 mm capable of preventing the dropping phenomenon without a large pressure loss; the passage volume is limited to 1.0 cm3 or smaller within the range of 70-115 mm of the nozzle main body length; and moreover, the hole diameter of the passage at the front end of the above-mentioned nozzle main body is set within the range of 0.9-1.2 mm for liquid crystal polymer resin, and within the range of 1.2-1.5 mm for polyphenylene-sulfide resin.
Moreover, the present invention is such that the diameter of the rear end hole of the above-mentioned passage is limited within 4-6 mm; the gradient up to the tip end hole is set within the range of {fraction (1/16.5)}-{fraction (1/33)}; and the passage wall is mirror-finished to prevent the resin from staying therein, and also the gradient of the nozzle port of the above-mentioned nozzle tip is set to ⅕-{fraction (1/15)} which is steeper than that of the passage, so that the solidified resin in the above-mentioned nozzle port is thoroughly drawn out and removed.
Further, according to the present invention, a heat insulating board with a through-hole of the same diameter at the center as the diameter of the front end hole of the above-mentioned passage and the internal side diameter of the above-mentioned nozzle port is arranged between the front end face of the above-mentioned nozzle main body and the above-mentioned nozzle tip.